1. Technical Field
This invention relates to a rubber crawler in the form of an endless belt having an inserting joint structure wherein a number of parallel reinforcing cords as main cords having ends directing in both directions are embedded in a rubber crawler, the ends of the parallel reinforcing cords are connected by being inserted between each other, and a process for manufacturing the same.
2. Background Art
In general, a number of reinforcing cords as reinforcing cords (main cords), e.g., coated with a rubber are embedded in a rubber crawler at an approximately intermediate part with respect to the thickness thereof, for increasing the rigidity of the rubber crawler in the form of an endless belt and increasing the driving force. Such rubber crawler is manufactured by maintaining main cords between an unvulcanized outer peripheral rubber on an ground contacting side and an unvulcanized inner peripheral rubber on an inner peripheral side, and forming the rubber in this condition into a rubber crawler in the form of an endless belt by the vulcanization with heat applied thereto.
FIG. 7 shows an embodiment of such structure. As shown in the figure, main cords 33 are structured as a belt like material having a predetermined length, coated with a rubber. Both ends 33A and 33B are inserted between each other, whereby a superimposed connection S′ is formed in an endless belt shape. Accordingly, an end of the superimposed connection S′ of the main cords 33 at the ground-contacting outer periphery shows a certain behavior, that is, the end tends to spring up to an outer peripheral side because tension is applied to the rubber crawler main body 31 and bending causes by winding on a driving sprocket. As a result, it is possible that a locational stress is generated, and that cracks generate and rapidly grow.
Various technologies are proposed, for preventing the generation of such locational stress at the superimposed connection S′ of the main cords 33. For instance, Japanese Kokai Publication 9 (1997)-249164 discloses that an end part of a steel cord connected by superimposing is formed to have a flat shape by untwisting the end part for a predetermined length, and that the thus treated end part is incorporated in the crawler main body made of a rubber material by embedding the end part therein. By this structure, the bending rigidity at the end part of the steel cord is decreased, the locational stress at a non-adhered part thereof is dispersed, and the spring up behavior is decreased. Thus, the end part of the steel cord has an improved durability.
In such conventional structure of the superimposing connection of the main cord, however, it is necessary to form the end part of the main cords such as steels cord to have a flat shape by untwisting the end part over a predetermined length. The process for this operation requires labor and time. Furthermore, it tends to cause ununiformity among constituent cords. Moreover, it is possible that the end part of the main cords get untied by a rubber flow during vulcanization of an unvulcanized rubber by application of heat, even if the thus specially configured end part of the main cords are put in a proper order and placed in an unvulcanized rubber. As a result, each constituent cord in the rubber crawler after vulcanization and forming receives uneven (non-uniform) tension. Therefore, it is possible that some cords are broken.
Under considering such situation, the applicant of the present invention proposed a rubber crawler as shown in FIG. 8 (Japanese Patent Application 11-160356) in order to maintain the uniform revolution of the rubber crawler and to improve the durability thereof. For this purpose, the bending rigidity of a superimposed connection in the rubber crawler is lowered, which is in the form of an endless belt where main cords having the superimposed connection are embedded. A brief explanation will be made about the above-mentioned technology. A number of reinforcing cords 12 as main cords are wound in a predetermined winding direction, and superimposed connections 12-1 and 12-2 are formed when viewed from a lateral side (in the direction of an arrow). Namely, the closest first cord is provided in a wound condition for one round from a beginning end 12-1 to a terminal end 12-2, thereby forming a superimposed connection where both ends are superimposed to one another when viewed from a lateral side.
In other words, the beginning end 12-1 and the terminal end 12-2 are provided on the same plane. All of the reinforcing cords are arranged in the same way. Accordingly, a rubber crawler in the form of an endless belt is structured by an inserting joint structure where a number of main cords are inserted and connected between each other.
By the proposal of the rubber crawler having the inserting joint structure which is formed in such condition where the number of main cords are inserted between each other in an identical plane, the bending rigidity of the connection part of the main cords is decreased, the revolution state is uniformly maintained, and hence the durability is increased.
However, a number of cords 12 as main cords are embedded in a rubber crawler has an inserting joint structure which by winding the cords in a predetermined winding direction, and inserting the cords between each other so as to locate both ends approximately on a same plane when view from a lateral side. In this case, it is inevitable that a widthwise shift generates between lateral ends 12A on one side and lateral ends 12B on the other side.
For clarification, the above configuration will be explained by referring to three reinforcing cords 12. Lateral ends 12A3, 12A2 and 12A1 on one side are successively inserted between lateral ends 12B3, 12B2 and 12B1 on the other side so as to arrange the one side ends on the other side ends. Accordingly, the outermost reinforcing cords 12A1 and 12B3 on both sides of the inserting joint are positioned with a shift by 1 pitch as a cord placing pitch. As a result, the spaces on both sides of the rubber crawler 11 are differently produced (b1>b2 in the figure). In this case, it is possible that the rubber crawler has a problem as to driving performance at a high speed. There is also such a risk that the narrower end of the rubber crawler is subjected to edge breakage by the use for a long period of time.